1.5A, 280kHz, Boost Regulator
LM5171
In selecting appropriate values for the slope compensation network, the designer is advised to choose a
convenient capacitor, then select values for R2 and R3 such that the amount of slope compensation added is 100
mA/μs. Then R2 may be increased or decreased as necessary. Of course, the series combination of R2 and
R3 should be large enough to avoid drawing excessive current from VSW. Additionally, to ensure that the control
loop stability is improved, the time constant formed by the additional components should be chosen such that
1- D
R3C3<
fSW
Finally, it is worth mentioning that the added slope compensation is a trade-off between duty cycle stability and
transient response. The more slope compensation a designer adds, the slower the transient response will be,
due to the external circuitry interfering with the proper operation of the error amplifier.
Soft−Start
Through the addition of an external circuit, a soft−start function can be added to the LM5171. Soft−start
circuitry prevents the VC pin from slamming high during startup, thereby inhibiting the inductor current from rising
at a high slope.
This circuit, shown in Figure 31, requires a minimum number of components and allows the soft−start circuitry
to activate any time the SS pin is used to restart the converter.
VIN
VCC
SS
SS
D1
C3
4uA
Test
Q
Test
VC
R1
C2
C1
Figure 31. Soft-Start
Resistor R1 and capacitors C1 and C2 form the compensation network. At turn on, the voltage at the VC pin
starts to come up, charging capacitor C3 through transistor Q, clamping the voltage at the VC pin such that
switching begins when VC reaches the VC threshold, typically 1.05V
Therefore, C3 slows the startup of the circuit by limiting the voltage on the VC pin. The soft−start time
increases with the size of C3.
Diode D1 discharges C3 when SS is low. If the shutdown function is not used with this part, the cathode of
D1 should be connected to VIN.
Calculating Junction Temperature
To ensure safe operation of the LM5171, the designer must calculate the on−chip power dissipation and
determine its expected junction temperature. Internal thermal protection circuitry will turn the part off once the
Dec. 2010 - Rev. 1.2.1
- 17 -
HTC
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
